The present invention relates to a rotation transmission device for selectively transmitting power from input to output members of a drive train.
JP patent publication 2003-120719A discloses a rotation transmission device for selectively transmitting driving torque to the front, auxiliary drive wheels of a four-wheel drive vehicle of the front-engine, rear-drive layout.
This conventional rotation transmission device includes an outer ring coupled to a propeller shaft, an inner ring coupled to a pinion shaft of a differential, and a two-way clutch mounted between the outer and inner rings. An electromagnetic clutch is provided adjacent to the two-way clutch to selectively engage and disengage the two-way clutch. When the two-way clutch engages, the outer ring and the inner ring are coupled together, so that the rotation of the outer ring is transmitted to the inner ring.
The two-way clutch includes a cylindrical surface formed on the inner periphery of the outer ring and cam surfaces formed on the outer periphery of the inner ring. The cylindrical surface and the cam surfaces define wedge spaces having narrow circumferential ends. Rollers are mounted between the cam surfaces and the cylindrical surface and are retained by a retainer. Relative rotation of the retainer and the inner ring causes the rollers to engage between the cylindrical surface and the cam surfaces. A switch spring is mounted between the inner ring and the retainer to bias the retainer to its neutral position where the rollers are disengaged from the cylindrical surface and the cam surfaces.
The electromagnetic clutch includes an armature rotationally fixed but axially movable relative to the retainer, a cylindrical rotor guide connected to the outer ring at its open end and covering the armature, a rotor inserted in the rotor guide from its open end and axially facing the armature, and an electromagnet axially facing the rotor. When the electromagnetic coil of the electromagnet is energized, the armature is attracted to and pressed against the rotor. The armature thus begins to rotate together with the outer ring and relative to the inner ring. The rotation of the armature relative to the inner ring causes the rollers to engage the cylindrical surface and the cam surfaces.
If the magnetic field produced when the electromagnetic coil of the electromagnet is energized to attract the armature to the rotor leaks through the rotor guide to the outer ring, the electromagnetic force to attract the armature decreases. This in turn reduces the torque transmitted from the outer ring to the armature, thereby making it difficult to reliably engage the two-way clutch.
To avoid this problem, that is, to prevent leakage of the magnetic field, conventional rotor guides are typically made of non-magnetic materials such as aluminum.
In such conventional rotation transmission devices, the rotor and the rotor guide are made of materials having different coefficients of linear expansion. Thus if the rotor is pressed into the rotor guide, when the rotor and the rotor guide are heated and expanded, there will not be enough interference left for the rotor to remain secured to the rotor guide.
Similarly, the rotor guide and the outer ring are made of materials having different coefficients of linear expansion. Thus, if the rotor guide is pressed on the outer periphery of the outer ring at its open end, when they are heated and expanded, there will not be enough interference left for the rotor guide to remain secured to the outer ring.
It is therefore necessary to provide, between the rotor 50 and the rotor guide 51 (see FIG. 5), means for preventing relative rotation, such as a protrusion 52 formed on the outer periphery of the rotor 50 and received in a cutout 53 formed in the rotor guide 51 at its end, and means for preventing axial movement, such as a snap ring 55 engaged in a groove 54 formed in the inner periphery of the rotor guide 51 at its end. It is also necessary to provide such means between the rotor guide and the outer ring. These means complicate the structure of the rotation transmission device and thus increase its cost.
The snap ring 55 makes it impossible to increase the axial length of the rotor 50, and thus the length of the portion of the rotor 50 axially overlapping with the electromagnet 56. It is therefore impossible to increase the magnetic attraction force for attracting the rotor 50.
An object of the present invention is to provide a rotation transmission device of the abovementioned type in which the rotor guide and the rotor can be coupled together with a simple structure and at a low cost and which includes means for increasing the magnetic attraction for attracting the armature to the rotor.